Hydrogen is attracting attention as an energy source to cope with environmental problems. That is, hydrogen is attracting attention as fuel of a fuel cell, fuel directly burned in an internal combustion engine, or the like. This is because of a characteristic that only water is generated even by burning hydrogen, and an energy density per weight is high. In addition, hydrogen can be easily obtained by electrolyzing water using electric energy such as a solar cell.
Meanwhile, since hydrogen is easily exploded by sparking or the like when it is mixed with oxygen, it is necessary to reliably detect leakage from a container such as a reservoir. That is, since hydrogen has a possibility of explosion when the concentration in the air exceeds 4%, a sensor for detecting this concentration with easiness in mass-production is demanded.
For this reason, as discussed in Patent Document 1, a hydrogen sensor has been developed, in which a hydrogen reaction catalytic layer is formed on a quartz crystal plate, and hydrogen is oxidized by the effect on the catalytic layer to generate heat, so that a temperature of the quartz crystal plate increases to change a natural frequency. The hydrogen concentration is measured by detecting the change of the natural frequency.
In the technique discussed in Patent Document 1, a hydrogen reaction catalytic layer is formed on a surface of a quartz vibrator, and a mass-production method is already established in the art, so that high practicality can be obtained.
In the technique discussed in Patent Document 2, the measurement value is stabilized, compared to the technique of Patent Document 1. That is, a hydrogen reaction catalytic layer is formed on one side or both sides of a fourth region of the quartz plate, and a hydrogen non-reactive layer is formed similarly in the fifth region of the surface of the quartz plate. In addition, hydrogen is oxidized by the hydrogen reaction catalytic layer to generate heat of combustion, and a change of the natural frequency of the first quartz vibrator of the quartz plate caused by the heat of combustion is measured on the basis of the natural frequency of the fifth region of the quartz plate, so that the hydrogen concentration is measured on the basis of the change of the natural frequency.